The leaf area index (hereinafter, called as LAI) represents the sum of areas of leaves that overlap in a certain direction (e.g. in a vertical direction) per unit area in a plant community or the like in croplands or in forests. The LAI is used as an index to grasp plant growth or cultivation condition in greenhouse cultivation or the like.
As a method for measuring the LAI, for instance, there is proposed an indirect measuring method using an illuminance sensor. In this method, for instance, illuminances are measured in an upper region and in a lower region within a plant community to estimate the LAI based on the measured illuminances thereof.
Further, as a related technique, PTL 1 describes a method, in which reflectors arranged in a plant community are photographed, and illuminances are calculated based on color information of the reflectors to estimate the LAI.
Furthermore, PTL 2 describes the LAI indirect measuring method. In the method described in PTL 2, an indirect measuring system photographs an image of a predetermined area with respect to each of near infrared light and red light, using a wide angle lens and an electronic element. Then, the indirect measuring system obtains a luminance value with respect to each of the near infrared light and the red light in each of segment areas obtained by dividing the predetermined area. Then, the indirect measuring system obtains a ratio of luminance values between the near infrared light and the red light in each of the detail areas, estimates a relative amount of solar radiation based on the ratio of luminance values, and obtains the leaf area index based on the relative amount of solar radiation.